Wall stack up opinions – using rigid foam as air barrier between double stud wall?
I’m in climate zone 6A looking to build a partition wall between an attic bonus room. One side of the bonus room will remain as cold storage, and the other side will be heated by minisplits and resistance backup. I’m looking at building a double 2×4 wall separated by rigid foam. The exterior wall would be framed 24″ o.c. to maximize insulation. There will be no plumbing or wires in the exterior wall. The interior framed 2×4 wall will be framed 16″ o.c. and will contain wiring, and will possibly be plumbed for a sink.
From cold to warm, here is what I’m proposing:
1. 5/8″ fire rated gypsum
2. R-15 high density batt insulation installed in 2×4 wall framed 24″ o.c.
3. 1″ polyiso insulation detailed as air barrier
3. R-15 batt insulation installed in 2×4 wall framed 16″ o.c.
4. 1/2″ gypsum
The foam “gap” at the top of the wall would be covered with gypsum or suitable fire barrier.
As this is a non-structural wall and utilizes standard dimension framing, I thought that batt insulation with the foam would result in better performance than using something like taped plywood for the air barrier. Just want to make sure there aren’t issues with the foam. Depending on what foam I use, the wall would be vapor open both ways, but I want to make sure it doesn’t become a condensing surface.
I’ve read the articles on calculating the min thickness of exterior foam, but what rules apply when the foam is sandwiched? If this works it seems like a great wall design, as the assembly should come in at approx R-26 at a wall thickness of about 9″.
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